Automobiles which can operate on alternative fuels, such as arbitrary mixtures of alcohol and gasoline, are indicative of future trends and, in fact, will soon be required by law in certain regions. For proper engine operation it will be necessary to measure the ratio of alcohol-to-gasoline within the fuel mixture which is being injected into the combustion chamber. Since the automobile may be filled with gasoline at one instance and an alcohol-containing gasoline mixture of up to about 85% methanol at the next, and because alcohol and gasoline can physically separate in the gas tank, this ratio may change very rapidly over a few minutes or even faster. Therefore, it is necessary that this ratio be determined continuously.
A variety of techniques have been previously proposed for making these on-board measurements of the alcohol content within the fuel mixture for control of the engine parameters. Typically, these methods have measured various properties of the gasoline mixture, including the dielectric constant, thermal conductivity, index of refraction, change in the speed of sound through the mixture and microwave absorption. However, these methods tend to be prohibitively expensive for widespread use or the measuring techniques involved are inherently problematic since they tend to be strongly dependent on temperature and/or the detailed properties of the gasoline used. Further, as an exacerbation of these shortcomings, the composition of a particular gasoline mixture may vary considerably even within a single name brand. Therefore, these methods have failed to provide the reliability required for automotive engine control applications.
An alcohol sensing device based on infrared spectroscopy methods would generally avoid the problems associated with these previous methods, including the strong dependence on temperature and/or the gasoline composition. This is because infrared spectroscopy is an analytical technique which measures the relative absorption of various infrared wavelengths by a particular specimen and is thereby dependent on the molecular constitution of the specimen. A sensor for determining the alcohol content in gasoline which utilizes such an infrared absorption technique is disclosed in U.S. Pat. No. 4,594,968 to Degobert et al, entitled "Process and Device for Determining the Composition of an Alcohol-Petrol Mixture, Adapted to the Automatic Regulation of Engines Fed with Fuel Mixtures Having a Variable Alcohol Content" issued, Jun. 17, 1986. However, there are many drawbacks associated with the use of this alcohol sensor, even though it utilizes the preferred infrared spectroscopy measurement techniques.
Degobert et al measure the alcohol content of the fuel mixture by determining the infrared absorbance of the fuel in the wavelength range between 0.7 and 1.7 micrometers. However, a reference measurement must first be made so that the intensity of the transmitted light through the fuel mixture can be referenced to the intensity of the original light source, for determination of the amount of absorbance. Degobert et al propose that the light beam be split, with one beam passing through an alcohol or gasoline/alcohol reference cell with known composition and the other beam passing through the fuel to be measured. This setup leads to several practical problems.
The Degobert et al system utilizes a beamsplitter, two sets of windows and two different detectors corresponding to both the reference and measuring cells. If any of these components change with time, which is extremely likely to occur particularly in the automobile environment, the signal from the device will be in error. For example if the inside of the measuring detector window becomes covered with a film from the fuel but the reference detector window stays relatively clean, then the measuring detector will sense relatively less light and the sensor will calculate a higher than correct alcohol content. The device fails to deal satisfactorily with the possibility that one window may become dirtier than the other.
In addition, the beamsplitter may become dirty in a way which will affect one light path more than the other, which again is a definite possibility in the dirty environment of an automobile. Also, it is extremely difficult to maintain the integrity of optical components which are exposed to flowing gasoline, as is the situation with the device of Degobert et al. For these reasons it is clear that it would be desirable to provide a sensing device which does not utilize a beamsplitter which duplicates optical paths and components for both the measuring and reference fuel sample cells, and which thereby avoids the shortcomings of the prior art.
Further, although this type of device proposed by Degobert et al utilizes infrared absorption spectroscopy, it is still strongly dependent on temperature due to the nature of its detection system. Within the engine environment of an automobile, the temperatures may fluctuate greatly over a wide range from about -40.degree. C. up to about 120.degree. C., making it difficult in practice to maintain the two relatively large detectors and sample cells of this device at identical temperatures unless they are independently thermostated. If the detectors are at different temperatures, the absorbance measurements will give erroneous results. The output voltage of the LED source is also temperature sensitive and would therefore have to be thermostated to ensure reliable results. Lastly, the infrared absorption coefficient for alcohol is temperature dependent, thereby requiring two separate temperature measuring devices (one for the fuel measurement and one for the reference measurement). These various thermal outputs must all be incorporated into the algorithm used for determining the air-fuel ratio in order to ensure an accurate measurement.
It is clear that these requirements all add substantially to the complexity and cost of the device. Therefore, it would be desirable to provide an alcohol sensor for determining the alcohol content in a fuel mixture for use in an automobile environment, which utilizes infrared absorption spectroscopy techniques but which alleviates the many shortcomings associated with the previously proposed devices. In particular, it would be desirable to provide such an alcohol sensor which does not require the use of a beamsplitter for duplicate sample cells, and which is not strongly dependent on temperature effects or the particular fuel mixture composition.